In P6, WBS codes are set up to organize activities by Work Breakdown Structure. Typically the WBS codes are created prior to all activities being added in the P6 schedule. If there are no activities assigned to a WBS code, that WBS code is considered empty. WBS levels are imported into Synchro as Summary Tasks. Summary Tasks in Synchro are required to have sub-tasks, which is why the empty P6 WBS levels are skipped during import and Synchronisation. If, later on, activities are added under these WBS levels in P6, they will import into Synchro.